In our culture's case, in general the amount each one ate doesn't matter -- whether one had steak and the other just a salad, the salad person can very well still go on and insist on paying for everybody else. Nobody would care about "a 15% to 30% difference in cost" (the person paying for everybody wont feel it's a burden, and the persons having their meal paid, wont feel like imposing).
It's not about achieving some perfect balance or fairness in any particular meal -- people genuinely want to treat others, and enjoy taking turns doing so.
It's only longer term one would care for patterns of behavior (the person who after so many meals with others never volunteers to pay etc).
If we're splitting a bill, I insist on this, because some of my friends don't drink alcohol, and some do.
(I drink.)
That's how it's in my country for all classes, poor or rich.
When friends go out, someone and pays for all the others (and someone might ask to chip in or pay themselves, but the first one doesn't take no for an answer).
Next time you see each other, the other person can pay, but no one keeps any very specific tally ("I paid 3 times, you paid 1") -- that would be considered tacky and cheap. That said, if someone systematically over many meetings never takes initiative to pay that the rest start considering them a cheapskate (but never giving specific tally or anything -- it's more from the feeling "never remember you paying").
Anybody insisting on splitting the bill, or obsessing over who ordered what, is also seen as a cheapskate.
That's how it usually goes among people with regular income. Now, sometimes, when the company is comprised of people that might be students, unemployed, etc, they more often agree to split the bill. But in this case too, obsessing over who bought what is considered rude, and usually it's just a simple bill/persons split that's paid by all.